Field of the Invention
The present invention relates to absorbent sanitary articles and regards a method and an apparatus for producing an absorbent structure for absorbent sanitary articles.
Description of Prior Art
The designation “absorbent sanitary articles” generally indicates articles such as, for example, diapers and pull-ups for infants and children, incontinence pads, sanitary napkins for women, and similar articles, intended to absorb body fluids.
These sanitary articles typically have a layered structure comprising an outer sheet or backsheet impervious to liquids, an inner sheet or topsheet permeable to liquids and intended to be placed in contact with the user's skin, and a central absorbent insert or core that has the function of capturing and storing body fluids.
In the first industrial diapers that appeared on the market in the late 1950s, the absorbent core was mainly composed of cellulose fluff with different shapes and thicknesses. Subsequently, in the second half of the 1980s the tendency developed to add superabsorbent granular materials—with a high degree of fluid retention- to the cellulose fluff.
These superabsorbent materials are known by various names such as, for example, SAP (acronym for Super Absorbent Polymer) or AGM (acronym for Absorbent Gelling Material). In most cases, hydro-gelling materials are used, which are capable of absorbing and capturing the liquid in a mostly stable manner.
By way of reference, one gram of cellulose fluff is capable of absorbing 8-10 grams of 0.9% saline solution, but it only retains a very small fraction of it, typically 2-3 grams. One gram of superabsorbent material is able to absorb about 50 grams of saline solution and to retain about 30 grams of it after centrifugation (EDANA method 441.1-99) or about 22 grams under a load of 0.7 psi (EDANA method 442.1-99).
In view of these characteristics, the idea of producing an absorbent core for sanitary articles composed primarily, if not exclusively, of superabsorbent material, certainly appears attractive.
Absorbent structures composed almost exclusively of superabsorbent material are known, and are described in numerous patent documents such as U.S. Pat. Nos. 4,600,458, 4,658,914, 4,681,577, 4,685,914, 5,643,238, 593,650 and WO-A-2005/004939.
One of the major difficulties in producing absorbent structures without fluff is that of combining together the integrity and the absorption capacity of the liquid of the structure. An absorbent core without fluff should have very high values of integrity and of absorption capacity. In reality, what happens is that if priority is given to one of the two characteristics, the other consequently results as being penalized, or rather a high absorption capacity is achieved at the price of an insufficient integrity of the structure and vice versa.
Methods and apparatus have already been proposed that have the object of penetrating granular material within a layer of highly voluminous fibers, in order to produce an absorbent structure without fluff. For example, WO2013/153235 describes a method for producing an absorbent structure comprising at least one fibrous non-woven layer having an empty volume configured to be penetrated by superabsorbent particles. The superabsorbent particles are dispersed into the fibrous layer by means of vacuum and vibrations.
The technology which provides the penetration of superabsorbent particles into a fibrous layer by means of vacuum and vibrations is not sufficiently effective because, in practice, it does not allow a deep penetration of the superabsorbent material granules into the fibrous layer and, consequently, does not allow the distribution of high amounts of superabsorbent material into the fibrous layer.
EP-A-1526214 describes an electrostatic method for the penetration of superabsorbent powder within a fibrous layer. The powder and the fibrous layer are placed between electrodes, which are connected to the poles of an alternating current generator. The electrodes form an electrostatic field with a voltage gradient in the order of 0.10-20 kV/mm.
In practice, the electrostatic penetration technology provides better results than the penetration technology with vacuum and vibrations. However, the electrostatic technology is affected by various drawbacks, including the high length of the electrostatic tunnel (in the order of 10 meters), the high cost and safety issues related to the risk of subjecting inflammable material to a high electrical voltage (up to 20 KV).
EP-A-0540041 describes a method for forming a super-absorbent composite material, which comprises a step of hydraulic needling of a sheet of non-woven fabric to increase its distribution properties of the liquids, and to introduce dry superabsorbent material in intimate contact with at least one surface of the hydraulically-needled fibrous sheet. This document describes that the composite sheet may be mechanically softened by calendering, opening, embossing, differential ironing, etc.